Virtual sorbent bed systems and methods of using same

ABSTRACT

Virtual sorbent bed systems and methods for receiving contaminants from a waste stream are presented. In an embodiment, the virtual sorbent bed system comprises an outlet for introducing into the gas stream a material capable of receiving contaminants, a first charged DC electrode oriented substantially peripheral to the gas stream and normal to the flow of the gas stream; a second charged DC electrode oriented substantially peripheral to the gas stream and normal to the flow of the gas stream, wherein the first DC charged electrode and the second charged DC electrode cooperatively generate a first electric field that imparts a drift velocity to the material; and a plurality of charged AC electrodes oriented substantially peripheral to the gas stream and normal to the flow of the gas stream, wherein the charged AC electrodes generates a second electric field that imparts additional three-dimensional motion to the material.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of U.S. Provisional PatentApplication No. 60/576,334, filed on Jun. 1, 2004, the disclosure ofwhich is herein incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to chemical technologies. Morespecifically, the present invention relates to virtual sorbent bedsystems and methods of using same.

Mercury has been recognized as a serious pollutant of concern due to itstoxic and bioaccumulative properties. Trace amounts of mercury can bemagnified up the aquatic food chain hundreds of thousands of times,posing a potential risk to humans and wildlife that consume contaminatedfish. In human beings, mercury adversely affects the central nervoussystem—the brain and spinal cord—posing a significant risk to developingchildren.

The U.S. EPA has created new regulations for the emission of mercury.The impending mercury emissions regulations will most directly affectmunicipal incinerators, medical-waste incinerators, and coal-burningboilers of electric utilities. These are the largest sources of mercuryemissions in the U.S., each accounting for roughly one-third of thetotal amount of mercury released in the U.S.

Municipal and medical-waste incinerators have specific characteristicsthat are conducive to controlling mercury emissions. Generally, theexhaust streams of both municipal and medical-waste incinerators aresmall and contain relatively high concentrations of mercury. Thesecharacteristics allow conventional exhaust cleaning methods toeffectively remove mercury. In particular, 70% of the mercury in theexhaust of municipal and medical-waste incinerators is in the form ofmercuric chloride (HgCl₂), which is easily removed by wet scrubbing anddry absorption processes. The characteristics of municipal andmedical-waste incinerators allow mercuric chloride (HgCl₂) to form.Because plastic comprises a large percentage of the wastes destroyed inincinerators, an ample source of chlorine is available for the hightemperature oxidation of elemental mercury (Hg⁰) to mercuric chloride(HgCl₂).

Compared to municipal and medical-waste incinerators, the removal ofmercury from the exhaust of coal-burning boilers of electrical utilitiesis more complex. Coal contains only trace amounts of mercury, 1-15 partsper billion, by weight. However, although coal contains only traceamounts of mercury, in 1997 combustion of over 900 million tons of coalreleased 50 tons of mercury into the environment. Compared to municipaland medical-waste incinerators, the typical exhaust gas stream from acoal-fired boiler is very large. The mercury in the exhaust ofcoal-burning boilers can exist in both physical forms (vapor andcondensed) and in both oxidation sates (elemental (Hg⁰) and oxidized(HgCl₂)). The total concentration of mercury and its distribution amongthe various forms and oxidation states initially depends on the detailsof the combustion process and the rank of the origin of the coal.However, these distributions are dynamic, shifting with changing gastemperature and gas composition throughout the exhaust train. As no twocoal-fired boilers have identical configurations, the evolution ofmercury in the post-combustion environment is virtually unique to eachfacility. Consequently, controlling mercury emissions from coalcombustion is extremely difficult due to the large degree of variabilityand uncertainty in the phase, state, and concentration of mercuryemitted from different facilities.

The electric utility industry is largely unprepared to reduce mercuryemissions. There is no feasible commercial technology available forcontrolling mercury emissions from coal-fired boilers. Prior artattempts at mercury emission control technologies, such as U.S. Pat. No.6,699,440 to Vermeulen, focus on fixed bed adsorption, requiring thatthe mercury-laden flue gas pass through a layer of powdered sorbentdeposited on a fabric filter. As 90% of coal-fired boilers do not havesuch fabric filers installed, such an approach constitutes aprohibitively expensive retrofit for many operators. Installing fabricfilters would also create increased pressure drop in the waste gasstream, entailing additional costs to install downstream induced draftfans, as well as reinforcement of upstream ductwork to support thegreater pressure differential. These issues create a high projected costfor reducing mercury emissions. Under contemporary pollution controltechnology, a 90% reduction in mercury emissions is projected to costthe electric utility industry from $6 billion to $15 billion annually.

It is therefore desirable to provide an efficient and cost-effectivetechnology for removing heavy metals and other chemicals from waste gasstreams.

SUMMARY OF THE INVENTION

The present invention generally relates to virtual sorbent bed systemsthat provide for an efficient and economical way for receiving (e.g.adsorbing, absorbing, contacting, mass transferring) various compoundsfrom waste gas streams.

In an embodiment, the present invention provides a system thatcomprises: at least one outlet for introducing a material into the gasstream, wherein the material is capable of receiving the contaminantfrom the gas stream; at least a first charged DC electrode; at least asecond charged DC electrode, wherein the first DC charged electrode andthe second charged DC electrode cooperatively generate a first electricfield that imparts a drift velocity to the material; and at least onecharged AC electrode, wherein the one charged AC electrode generates asecond electric field that imparts additional motion (e.g. two or threedimensional motion) to the material.

In an embodiment, the material is electrically charged prior to enteringthe gas stream.

In an embodiment, the first charged DC electrode and the second chargedDC electrode have a different voltage.

In an embodiment, the outlet comprises the first charged DC electrode.

In an embodiment, the second charged electrode comprises a plate capableof receiving and collecting the material.

In an embodiment, the at least first charged AC electrode comprises aplurality of charged AC electrodes, each charged AC electrode orientedsubstantially peripheral to the gas stream and normal to the flow of thegas stream, wherein each charged AC electrode generates a secondaryelectric field that imparts additional motion to the material.

In an embodiment, the at least one outlet comprises a plurality ofoutlets that are stacked.

In an embodiment, the at least one outlet comprises a plurality ofoutlets that are in series along the gas stream.

In an embodiment, the motion is periodic.

In an embodiment, the material is a solid material selected from thegroup consisting of a sorbent, a catalyst and combinations thereof.

In an embodiment, the material is capable of receiving a plurality ofcontaminants from the gas stream.

In an embodiment, the outlet is capable of injecting a liquid into thegas stream.

In an embodiment, the outlet is located upstream of the first charged DCelectrode.

In an embodiment, the injected liquid is selected from the groupconsisting of an ammonia solution, a urea solution, an aerosol andcombinations thereof.

In another embodiment, the present invention provides a virtual sorbentbed system comprising: a plurality of positively charged DC outlets forintroducing a material into the gas stream, wherein the material iscapable of receiving the contaminant from the gas stream and wherein thepositively charged DC outlets are oriented substantially peripheral tothe gas stream and normal to the flow of the gas stream; at least asecond negatively charged DC electrode located downstream of thepositively charged DC outlets and oriented substantially peripheral tothe gas stream and normal to the flow of the gas stream, wherein theplurality of positively charged DC outlets and the second negativelycharged DC electrode cooperatively generate a first electric field thatimparts a drift velocity to the material; and a plurality of charged ACelectrodes oriented substantially peripheral to the gas stream andnormal to the flow of the gas stream, wherein the plurality of chargedAC electrodes generate a second electric field that imparts additionalmotion to the material.

In an embodiment, the material is selected from the group consisting ofa solid material, a liquid material, a powdered material, an aerosol, asorbent, a catalyst and combinations thereof.

In another embodiment, the present invention provides a method forreceiving contaminants from a gas stream, the method comprising:introducing a material into the gas stream through at least one outlet,wherein the material is capable of receiving the contaminant from thegas stream; generating a first electric field from at least a first DCcharged electrode and at least a second charged DC electrode, whereinthe first electric field imparts a drift velocity to the material andwherein the first and second DC charged electrodes are orientedsubstantially peripheral to the gas stream and normal to the flow of thegas stream; and generating a second electric field from at least onecharged AC electrode oriented substantially peripheral to the gas streamand normal to the flow of the gas stream, wherein the second electricfield imparts additional motion to the material.

In an embodiment, the method comprises capturing or collecting thematerial after the material has removed the contaminant from the gasstream.

An advantage of the present invention is to provide a more costeffective and efficient system for receiving or removing contaminantsfrom a waste gas stream.

Another advantage of the present invention is to provide an efficientsystem for detecting biological contaminants in the air.

Still another advantage of the present invention is to provide a systemfor reusing sorbent thereby obtaining a cost-savings.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a schematic illustrating an end view of the virtual sorbentbed system in one embodiment of the present invention.

FIG. 1B is a schematic illustrating an top or plan view of the virtualsorbent bed system in one embodiment of the present invention.

FIG. 2 is a graph illustrating the comparison of the particletrajectories and normalized swept volume for particles subjected tohydrodynamic drag, electrostatic drift and electrodynamic oscillation.

FIG. 3 is a schematic illustrating constructions of injected sorbentresidence time.

FIG. 4 is a graph illustrating the radial distributions of property orconcentration differentials for an axisymmetric jet.

FIG. 5 is a graph illustrating normalized radial concentration gradientsfor four initial jet velocity ratios.

FIG. 6 is a chart illustrating the in-flight adsorption efficiencyversus the number of Teflon spheres.

FIG. 7 is a graph illustrating the comparison of in-flight and fixed bedadsorption.

FIG. 8 is a schematic illustrating an experimental model of the virtualsorbent bed apparatus in one embodiment of the present invention.

FIG. 9 is a schematic view of the virtual sorbent bed system in oneembodiment of the present invention.

FIG. 10 is a perspective view of an embodiment of the virtual sorbentbed system in one embodiment of the present invention.

FIG. 11 is a front view of the virtual sorbent bed system of FIG. 10 inone embodiment of the present invention.

FIG. 12 is a side or elevation view of the virtual sorbent bed systemshowing the outlets stacked in one embodiment of the present invention.

FIG. 13 is a top or plan view of the virtual sorbent bed system showingthe outlets in series in one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to chemical remediation technologies forreceiving (e.g. adsorbing, absorbing, contacting, mass transferring)various pollutants from emitted industrial gas streams. Morespecifically, the present invention relates to virtual sorbent bed(“VSB”) systems and methods of using same. In an embodiment, the VSBsystem generally comprises electrodes or any suitable electric fieldgenerators that produce electric fields (e.g. AC and DC) whichmanipulate the movement of a charged suspension of a sorbent powder toseparate contaminants such as heavy metals and other chemicals fromwaste gas streams.

Sorbent beds may be, for example, dense, charged suspensions of asorbent. The sorbent can be any suitable material, such as powderedactivated carbon, that is capable of being suspended or movable in gasstreams and capable of receiving heavy metals and chemicals from gasstreams. The dense, charged suspension can be bounded by mutuallyorthogonal AC and DC electric fields. It has been found that theapplication of electrostatic (DC) and electrodynamic (AC) forces on theparticles in the suspension causes them to trace sinusoidal pathsthrough the flowing gas. The continuous, sinusoidal relative motionbetween the suspended particles and flowing gas greatly enhancesgas-particle mass transfer as compared to the diffusive mass transferthat would occur within a suspension having no net charge. Yet, becausethe particles are suspended within the flowing gas, they induceeffectively no fluid pressure drop.

FIGS. 1A and 1B show a schematic of an end view and a top or plan view,respectively, of one general embodiment of the VSB system 20 adaptedspecifically for removing trace concentrations of mercury from coalcombustion exhaust. Suspended and charged sorbent or material 40 issuesinto the mercury-laden exhaust stream from at least one injector oroutlet 50 comprising at least one first charged DC electrode 52. Thecharged material 40 may comprise, for example, a solid powdered sorbentor a liquid material. The material 40 may be positively or negativelycharged or not charged at all. The arrow represents the direction of theairflow in FIG. 1B. The induced DC electric field between the firstcharged outlet 50 and a plate 60 comprising at least one second DCelectrode 62 exerts a constant coulombic force on the charged material40. In another embodiment, the plate 60 could be distinct from thesecond DC electrode(s).

The first charged DC electrode 52 and the second charged DC electrode 62have a different voltage thereby forming a direct current field betweenthe two charged sources. This field induces a constant electrostaticdrift velocity, normal to the gas velocity, drawing the charged material40 through and across the mercury-laden gas stream. In anotherembodiment, the first charged DC electrode could have a positive ornegative voltage and the second charged DC electrode could be the ground(i.e. 0 voltage). It should be appreciated that any suitable combinationof voltages/ground can be used for the first and second DC electrodes togenerate a potential difference and a direct current field between theelectrodes.

Similarly, charged AC electrodes 72 generate complementary AC electricfields that superimpose a sinusoidally varying electrodynamic driftvelocity that is orthogonal to both the gas velocity and theelectrostatic drift velocity. The cumulative effect of both electricfields is to impart a high degree of relative motion (e.g. two and threedimensional motion) between the gas and the particulate phases. Itshould be appreciated that the shape of the suspended material 40 in theFigures are for illustrative purposes only and are not intended torepresent the actual motion of the suspended material 40.

Alternatively, the embodiments shown in FIGS. 1A-1B may represent halfof the embodiments as depicted by the center line C.L. For example, theoutlets 50 and first DC electrode 52 may have on both sides the plate60, second DC electrodes 62 and AC electrodes 72 to double theflow-through gas volume.

In an embodiment, the VSB system 20 utilizes, for example, a gas solidmass transfer process that exploits the beneficial mass transfercharacteristics of suspensions. The relatively small temporal andspatial scales of dense and/or turbulent suspensions complicatecharacterization of their behavior. The VSB system 20, by virtue of itsexceptional control over the dispersed phase exerted by the dualelectric fields, allows existing mass transfer coefficients andcorrelations to be extended to dense and/or turbulent suspensions.

FIG. 2 illustrates the effect of gas-particle relative motion on masstransfer to the particulate phase. FIG. 2 depicts trajectories ofsorbent particles under three conditions: 1) subjected to hydrodynamicforces alone 12; 2) subjected to both hydrodynamic and electrostaticforces 14; and 3) subjected to hydrodynamic, electrostatic, andelectrodynamic forces combined 16. The superposition of hydrodynamic,electrostatic, and electrodynamic forces causes the particles to tracethe longest paths through the gas. Defining swept volume V_(S) as theproduct of particle path length and particle cross-sectional area, for aspecified particle diameter, the value of V_(S) will increase as theparticle path length increases. Defining a normalized swept volumeV_(S)/d_(p) (where d_(p) is the particle diameter) provides a means forcomparing the mass transfer enhancement exhibited by particles ofdifferent sizes as they are subjected to hydrodynamic, electrostatic,and electrodynamic forces.

In FIG. 2, for a representative particle size, charge, and gas velocity,the normalized swept volume V_(S)/d_(p) increases from 4 m² forhydrodynamic forces alone to 16 m² when hydrodynamic, electrostatic, andelectrodynamic forces are superposed, a four-fold increase. Assumingthat gas-particle mass transfer scales with V_(S)/d_(p), these resultssuggest that virtual sorbent beds should achieve four times greater masstransfer than uncharged suspensions. The differences in mass transferare even more striking if they are considered relative to a coordinatesystem moving with the gas. Such a coordinate system is more appropriatethan an inertial coordinate system for considering gas-particle masstransfer. If in this coordinate system, a modified swept volume (V*_(S))and modified normalized swept volume (V*_(S)/d_(p)) are defined, thenthe values of V*_(s)/d_(p) are 0 m² for hydrodynamic forces alone, 6 m²for both hydrodynamic and electrostatic forces, and 12 m² for combinedhydrodynamic/electrostatic/electrodynamic forces. In summary, imposingelectrostatic/electrodynamic forces produces a substantial performanceenhancement for mass transfer over uncharged suspensions.

In an embodiment, the method of receiving contaminants from a gas streamcomprises introducing charged powdered sorbent into the gas stream. Forexample, the outlet 50 can introduce the charged powdered sorbent as adense suspension initially contained within a low-velocity planar jet.This approach concentrates the suspension to enhance mass transfer andinhibits turbulent mixing of the sorbent-laden jet with itssurroundings, thereby minimizing jet mixing and its associated negativeimpacts on mass transfer within the sorbent suspensions.

In another embodiment, the VSB system 20 utilizes entrained or in-flightadsorption. In-flight adsorption occurs within flowing gas-sorbentsuspensions. In-flight adsorption offers larger gas-sorbent interfacialareas and longer periods of gas-sorbent contact. Sorbent particles insuspension more easily dissipate the heat of adsorption, eliminating therisk of carbon bed fires. Applied to coal combustion, the VSB requiresno expensive retrofits to install baghouse filters and induce minimalpressure drop in the gas stream. In an embodiment, the VSB system 20 canbe paired in series with an electrostatic precipitator. This would allowthe injected sorbent and fly ash to be collected separately so that theformer can be recycled and regenerated while also preserving the marketfor fly ash. In an embodiment, the VSB system 20 is highly flexible,allowing it to respond in real time to operational transients, fuelblending, fuel switching, and part-load operation. Unlike fixed sorbentbeds formed on fabric filters, the VSB system 20 can be completelyidled, becoming a transparent exhaust train component when conditionswarrant. Finally, in-flight and fixed bed adsorption for mercury controlneed not be mutually exclusive. Injecting a powdered sorbent toestablish a downstream fixed sorbent bed necessarily involves thecreation of a gas-sorbent suspension. Consequently, even where fixed bedadsorption is favored, in-flight adsorption can augment the performanceof the fixed bed and reduce rates of sorbent usage.

As shown in FIG. 3, dry sorbent injection for mercury capture typicallytakes the form of a powdered sorbent, suspended on high velocityaxisymmetric gas jet, and injected in a downstream orientation. A singleinjector lance or an array of lances may cause an “in-flight” or“entrained flow” residence time. This figure is intended to reflect theelapsed time between sorbent injection and sorbent filtration on adownstream fabric filter, typically between 0.5 and 2 seconds. However,this time value may be determined by dividing the distance betweeninjection and filtration by the mean velocity of the gas in the ductupstream of the injectors. Sorbent injection tends to concentrate thesorbent in the jet core where velocities remain substantially higherthan the surrounding medium for many nozzle radii downstream. The netresult is that sorbent in the jet core traverses short distances up to25% faster than the medium outside the jet. Solid or liquid mattersuspended within a gaseous jet tends to remain concentrated near the jetcore as the jet itself expands.

FIG. 4 illustrates the radial distribution of normalized propertydifferentials in an axisymmetric jet. The velocity difference U(r)between the jet and the surrounding medium at any radial locationdecreases with increasing radial distance from the centerline. Thespecies concentration C(r) within the jet decays less rapidly thanvelocity, indicating that material suspended within the jet undergoes aslower rate of radial spread and dispersion than the jet itself.

FIG. 5 presents four normalized radial concentration profiles at adistance of 18 m from a 1.25 cm injector orifice, representing fourjet-to-freestream velocity ratios (U₀/U₁). The primary differencebetween the different profiles is the higher rate of jet spreading, andthe attendant increase in jet cross-sectional area, with increasingvelocity ratio. Higher jet velocities produce wider jets at a givendownstream location. The radial growth of the jet leads to an inherentdecrease in injected species concentration as the jet volume increases.Species concentration decreases with increasing downstream distance,decaying as the inverse of jet diameter squared. However, dispersion ofmass transfer of the injected species does not keep pace with the jetspreading rate. The injected species remains concentrated about the jetcenterline, irrespective of the initial velocity ratio of the jet.

Under the proper conditions, trace gas adsorption within turbulentgas-sorbent suspensions could far exceed the adsorption efficiencies offixed sorbent beds on a per mass basis. These results (FIGS. 6 and 7)are based on the following experimental method. A cylindrical aluminumpressure vessel, mounted on a paint shaker, contains a set number ofTeflon spheres. After evacuating the cylinder, a 10 ppm mixture ofnitrogen gas and sulfur hexafluoride (SF₆) gas is added. A mass ofpowdered activated carbon is injected into the cylinder, forming asuspension. The paint shaker is then started, mechanically agitating thecylinder. The agitation of the cylinder by the paint shaker causesrandom motion of the Teflon spheres inside the cylinder, simulatingisotropic turbulence. After a set period of time, the paint shaker isstopped, the contents of the cylinder extracted, and the residualconcentration of SF₆ measured by Fourier transform infrared (FTIR)spectroscopy. The data showed that compared to the adsorption achievedwith no Teflon spheres in the cylinder, the addition of a single spheredoubles the adsorption efficiency. Adsorption efficiency increasesfurther as the number of Teflon spheres increases from 1 to 50.

Additional results provided a much more comprehensive evaluation of theinfluence of sphere diameter, agitation time, suspension temperature andmoisture content, injected sorbent mass, and sorbate concentration onadsorption within these turbulent gas-sorbent suspensions. Comparablefixed bed adsorption results were obtained by drawing the same SF₆/N₂gas mixtures through fixed sorbent beds containing the same amount ofsorbent that was injected for the in-flight tests. For the carbon massratios examined (120:1 to 6000:1), the fixed bed adsorption efficiencywas negligible. This demonstrates that, particularly at low carbon massratios, fixed bed adsorption is quite ineffective relative toturbulence-assisted in-flight adsorption. Given that powdered activatedcarbon costs on the order of $1 per pound, the opportunity to reducecarbon mass ratios translates directly into reduced operating costs.

FIG. 7 presents a more extensive comparison of in-flight vs. fixed bedadsorption (up to 25,000:1 carbon mass ratio) obtained using the sameexperimental method. The adsorption data associated with higher carbonmass ratios confirm the previous conclusion that in-flight adsorption isfar more effective than fixed bed adsorption at low carbon mass ratios.As carbon mass ratio increases, fixed bed adsorption performanceimproves, but still falls well short of the in-flight adsorptionperformance. Fly ash has been proposed as a potential sorbent “generatedin situ” because fly ash contains varying amounts of unburned carbonthat has a finite sorption capacity.

Electrostatic drift of particles within a suspension enhances masstransfer. When powdered activated carbon is injected upstream of anelectrostatic precipitator, up to 70% removal can be achieved within theelectrostatic precipitator. This suggests that the electrostatic driftvelocity imposed on the sorbent particles in the electrostaticprecipitator is the result of adsorption by fly ash and sorbent dustcake covering the plate electrodes within the electrostaticprecipitator. It was, however, shown that a representative Schmidtnumber (the ratio of dynamic viscosity, ν, and binary diffusioncoefficient, D_(ad)), Sc, for a mercury-laden flow through anelectrostatic precipitator is 12. For Sc=12, the mercury concentrationgradients are confined to a region very near the walls of theelectrostatic precipitator, making it impossible to support Fickiandiffusion of mercury from the bulk gas flow to the electrostaticprecipitator surfaces.

The VSB may have a dense suspension. As an example, consider 1 m³ of air(at standard temperature and pressure) containing 15 ppbw of elementalmercury. Treating this gas volume by uniformly dispersing powderedactivated carbon at a carbon-to-mercury mass ratio of 30,000:1 wouldyield a suspension having a minimum interparticle spacing of 3.4 mm (85particle diameters for 40 μm particles). By contrast, the VSB wouldconcentrate the same mass of sorbent into a dense sorbent layer (1 m×1m, H×W), the minimum interparticle spacing would decrease by 66% (1.1 m,28 particle diameters) compared to the uniformly distributed case.

It should be appreciated that the beneficial characteristics ofalternative embodiments of the VSB system can be extended to many otherprocesses involving mass transfer between a flowing gas and a solidmaterial. For example, catalytic gas treatment processes often employlarge, unwieldy, solid catalyst monoliths. In order to maximizegas-solid mass transfer, these monoliths often take the form of highsurface area honeycomb structures. Although such structures present avery large surface area for mass transfer, they also induce a largepressure drop within the gas flow. A VSB system would provide equal orgreater surface area for mass transfer without any induced pressure dropin the gas stream.

FIG. 8 illustrates an experimental scheme of an embodiment of the VSBsystem. For example, the experimental VSB system comprises two conjoinedrectangular plenums (e.g. Plexiglass) and fitted with gas connections attheir bases. Both plenums dispense gas flows vertically upward. Theleft-hand plenum dispenses the charged sorbent powder suspended in anitrogen gas stream that, under the influence of AC and DC electricfields, forms the VSB system. The right-hand plenum dispenses a nitrogengas stream containing trace concentrations of the sorbate species (e.g.elemental mercury, Hg⁰). The two gas streams are dispensed at a commonelevation plane and with identical velocities. The resulting parallel,co-flowing gas streams form a planar geometry that is diagnostically andanalytically convenient.

Nitrogen gas is mixed with charged powdered activated carbon (e.g. 30 μmmean particle diameter) that is fed into the left-hand plenum by astainless steel auger. The auger feed rate varies with input voltage,permitting the mass ration of carbon to sorbate (e.g. carbon-to-mercuryratio) to be varied from 10²:1 to 10⁵:1. A high voltage source incontact with the metal auger provides a mechanism for simultaneouspowder charging and feeding. A honeycomb flow straightener (not shown)at the exit plane of the right-hand plenum maintains a spatially uniformflow. Electronic flow controllers (not shown) meter the flow rates ofnitrogen into both plenums and to assure that the exit velocities ofboth flows are equal and representative of flue gas velocities (2-5 m/sin a precipitator). Preferably, the two flows are equal andrepresentative of exist velocity in order to prevent the development ofa shear layer at the interface between the two streams. The developmentof a shear layer would produce turbulent eddies that would interferewith the electrostatic/electrodynamic particle drift from the suspendedsorbent flow into the trace gas flow. The twin fluid flows are orientedupward to negate any effects of gravitation settling on the motion ofparticles.

Upstream electrical resistance heaters (not shown) provide optionalheating of both gas flows prior to their entering the two plenums. Gastemperatures typically range form 110 to 150° C. inside an operatingelectrostatic precipitator. Because powdered activated carbon adsorptioncapacity decreases with increasing temperature, how VSB performancevaries with gas temperature should be evaluated to optimize performanceconditions. Behavior similar to the preliminary results is expected,which showed that in-flight adsorption surpasses fixed bed adsorption ona per mass of sorbent basis even at elevated temperatures.

For mercury adsorption data, a permeation oven infuses a meterednitrogen gas stream of 44 ppb of elemental mercury (Hg⁰) in a processidentical to previous in-flight mercury investigations. For othersorbate data such as toluene and benzene, passing a nitrogen gas streamover a constant temperature liquid sorbate bath infuses it with anequilibrium concentration of sorbate vapor.

The performance of the VSB is measured in terms of adsorptionefficiency. Adsorption efficiency is defined as the percentage ofinitial sorbent that is adsorbed during the VSB process. Extractivemeasurements of the sorbate concentration downstream of the VSB, incombination with the known initial sorbate concentration of the gasstream entering the VSB, yields the absorption efficiency. Theexperimental test matrix provides the necessary data to correlate VSBperformance with gas temperature, moisture content, and velocity;sorbent charge and mass injection rate; electrostaticdrift-to-freestream velocity ratios; and AC voltage and frequency.

A Buck Analytic adsorption spectrometer measures elemental mercuryconcentrations in extracted gas samples down to single ppbconcentrations. Extracted gas samples are taken downstream of the VSB,eliminating the need to filter the extracted gas sample before it entersthe Buck analyzer. Both powdered activated carbon and high carbon flyash serve as sorbents; the fly ash may prove attractive if its generallypoor adsorption characteristics are mitigated by improved mass transferin the VSB and its low cost.

Doping the powdered activated carbon with fluorescent microspheres willallow direct imaging of select particle paths when the VSB suspension isilluminated with the appropriate frequency of UV light. Capturing suchimages using a digital camera will allow verification that the chargedVSB suspension is responding directly to the frequency and amplitude ofthe applied AC field. Any damping or phase lag of the sinusoidaloscillations, or any spatial variations in the amplitude or frequency ofthe oscillations of the particle's paths will be revealed throughanalysis of these images.

In another embodiment illustrated in FIG. 9, the VSB system 20 comprisesone or more openings, passages, vents, injectors or outlets 50 forintroducing the sorbent or material 40 into the gas stream, wherein thematerial 40 is capable of receiving a contaminant from the gas stream.Receiving a contaminant may refer to absorbing, adsorbing or contactingthe contaminant or may refer to the surrounding conditions (e.g. airpressure, air currents, temperature, material or contaminant motion)within the gas stream that cause or induce mass transfer from the gasphase to the solid or liquid phase of the material 40. For example, thefirst and second electric fields may facilitate the mass transferbetween a charged powdered solid material such as activated carbon andtrace amounts of gas species within the gas stream. Preferably, theoutlet 50 injects the charged material 40 into the gas stream in asheet-like manner so that the charged material covers a large volume inthe gas stream.

It should be appreciated that the material 40 may be any solid or liquidmaterial capable of receiving a contaminant from a waste gas stream. Forexample, the material 40 can be a solid material such as a sorbent,catalyst or combinations thereof. The sorbent can be powdered materialsuch as powdered activated carbon. Further, the contaminants in the gasstream may undergo reactions by contacting the catalysts. In addition,the material 40 may be capable of receiving a plurality of contaminantsfrom the gas stream.

The VSB system 20 also comprises at least a first charged DC electrode52, at least a second charged DC electrode 62 and at least one chargedAC electrode 72. The first charged DC electrode 52, the second chargedDC electrode 62 and the at least one charged AC electrode 72 may, forexample, be oriented substantially peripheral to the gas stream andnormal to the flow of the gas stream. The first DC charged electrode 52and the second charged DC electrode 62 cooperatively generate a firstelectric field that imparts a drift velocity to the material 40. One ormore charged AC electrodes 72 generate a second electric field thatimparts additional motion to the material. The additional motion may betwo or three dimensional motion and may be periodic such as sinusoidalmotion or square wave motion. The additional motion induced by thesecond electric field may more efficiently and effectively improve thefacilitation of mass transfer of the contaminants in the gas to thematerial 40. It should be appreciated that the first charged DCelectrode 52, the second charged DC electrode 62 and the at least onecharged AC electrode 72 may, be oriented in any suitable manner at, nearor away from the gas stream to achieve the same objective offacilitating mass transfer of the contaminants in the gas to thematerial 40.

In an embodiment, the material 40 is electrically charged prior toentering the gas stream. Generally, the charged material 40 is morereadily influenced and manipulated by the electric fields generated bythe DC and AC electrodes. Accordingly, the charged material 40 in theVSB system 20 undergo greater motion within the gas stream. It should beappreciated that the material 40 can be charged by any suitable methodknown to those having ordinary skill in the art.

In an embodiment, the first charged DC electrode 52 and the secondcharged DC electrode 62 have a different voltage. For example, the firstcharged DC electrode 52 can be positively charged and the second chargedDC electrode 62 can be negatively charged. Alternatively, the firstcharged DC electrode 52 and the second charged DC electrode 62 can havea voltage differential sufficient enough to cause a DC electric fieldbetween the first and second charged DC electrodes. In anotherembodiment, the first charged DC electrode 52 could have a positive ornegative voltage and the second charged DC electrode 62 could be theground (i.e. 0 voltage). It should be appreciated that any suitablecombination of voltages/ground can be used for the first and second DCelectrodes to generate a potential difference and a direct current fieldbetween the electrodes.

In an embodiment illustrated in FIG. 9, the second charged DC electrode62 can comprise a charged plate 60 capable of receiving or collectingthe material 40. For example, before the material 40 in the gas streamleaves the VSB system 20, some or all of it collects or amasses on theplate 60 because of the voltage differential between the first chargedDC electrode 52 and the second charged DC electrode 62.

In an embodiment, the VSB system 20 may have a voltage source 110connected to ground (not shown) and connected to an amplitude andfrequency controller 114. The size, shape, and configuration of thecontroller 114 and the voltage source 110 is can by any suitable foruse. The amplitude and frequency controller 114 is connected to one ormore AC electrodes 72 as shown in FIG. 9. The AC electrodes 72 arepreferably oriented longitudinally parallel to the flow of the gasstream, with the leading edge of the AC electrodes on the same plane asthe following edge of the charged injectors or outlets 54, on a planeperpendicular to the flow of the gas stream. The AC electrodes 72 can beconnected to the interior housing of a gas stream containment.

Each charged AC electrode 72 is individually be capable of generating asecondary electric field that imparts the additional motion to thematerial. For example, the AC electrodes 72 create an electric field offrequency and period as regulated by the amplitude and frequencycontroller 114 to facilitate the mass transfer between the material 40and the trace gas species to be removed from the gas stream. The ACelectrodes 72 can be made of any suitable conductive material such as,but not limited to, copper, aluminum, or steel. Preferably, the ACelectrodes 72 may have a curved cross-section along the short lengthonly, convex toward the gas flow; however other shapes can be used.

As illustrated in FIG. 9, in an embodiment, a motor (not shown)connected to a driveshaft 116 cooperates to drive an auger 118. Theauger 118 may be made of any conductive material. One end of the auger118 connects with a hopper 120 for supplying a material 40 such ascharged powdered solid material into a conduit 124. The hopper 120 canbe any suitable shape, size or configuration. The conduit 124 can alsobe of any suitable shape, size or cross-section, configuration andcontortion sufficient to facilitate the mixing and transmission of thepowdered solid material. The conduit 124 can be fed motive air by a fan122 which connects to the conduit 124 upstream of the material 40 feed.Motive air may also be supplied to the conduit 124 using a source ofcompressed air (not shown).

A voltage source 126 can be connected to the auger 118 to facilitatecharging of the material 40 prior to dispersal into the gas stream.Motive air mixes with the material 40 within the conduit 124.

In an embodiment illustrated in FIGS. 10-11, one or more outlets 50 cancomprise the first charged DC electrode 52 to form a charged injector oroutlet 54 thereby simplifying the VSB system 20. The charged groundplate 60 can be fixed to an interior housing of the gas flow andopposite the charged outlets. The leading edge of the ground plate 60can be on the same plane as the following edge of the charged outlets54, on a plane perpendicular to the flow of the gas stream. Preferably,the ground plate 60 is rectangular in shape; however, other suitableshapes can be used. The ground plate 60 can be connected to a ground andcan be made of any conductive material.

A collection trough 100 for solid or liquid materials can be positionedbelow the ground plate 60 to facilitate the collection of the material40. Preferably, the powder collection trough 100 is a channel, orientedto collect material 40 as it falls from the ground plate. Thelongitudinal axis of the trough 100 can be parallel to the direction ofgas flow as shown in FIGS. 10-11.

After any given period of time after the material 40 has collected onthe plate 60, the plate 60 can be tapped, hit or vibrated to cause thematerial 40 to fall into a powder or liquid collection trough 100. Thepowder or liquid collection trough 100 can accumulate the liquidmaterial or the solid material for subsequent re-use or recycling. Thiscan generate cost-savings through reuse of the sorbent material.

It should be appreciated that the outlet 52 can comprise any suitablemechanism for introducing material into the gas stream. Preferably theoutlet 52 has a wide opening, for example, capable of forming a denselayer of charged powdered material 40 in the gas stream.

As illustrated in FIG. 10, in an embodiment, the conduit 124 can connectto a plurality of charged outlets 54. The plurality of charged outlets54 are also connected to the voltage source 126. The charged outlets 54may be fixed to the interior wall of the housing of the gas streamoriented vertically, normal to the flow of the gas stream, spanning theheight of the gas stream. The cross-section of the leading edge of thecharged outlets 54 is preferably curved to prevent the formation ofturbulent eddies in the gas stream. The charged outlets 54 may beoriented to facilitate the dispersal of powdered solid material into thegas stream parallel to the flow of the gas stream, and at gas streamvelocity. The charged outlets 54 may be made of any suitable conductivematerial.

In an embodiment, the outlet 50 can comprise a plurality of outlets thatare stacked as illustrated in FIG. 12, or the outlet 50 can comprise aplurality of outlets that are in series along the gas stream asillustrated in FIG. 13. It should be appreciated that the embodimentsshown in FIGS. 12-13 may represent half of the embodiments as depictedby the center line C.L. For example, the outlets 50 and first DCelectrode 52 may have the plate 60, second DC electrodes 62 and ACelectrodes on both sides to double the flow-through gas volume.

In an embodiment, the outlet of the VSB system 20 may be capable ofinjecting a liquid into the gas stream. For example, the outlet oroutlets may be injectors or any suitable devices for injecting a liquidinto the gas stream. The liquid can be dispersed, for example, as anaerosol. Preferably, the injector or injectors for injecting liquid arelocated upstream of the first charged DC electrode at a distancesufficient to assure a largely dispersion and uniform liquiddistribution within the gas stream by the time the liquid in the gasstream reaches the charged electrodes. For example, the injected liquidcan be an ammonia solution, a urea solution, an aerosol and combinationsthereof.

In an alternative embodiment, the VSB system 20 comprises: a pluralityof positively charged DC outlets for introducing a material into the gasstream, wherein the material is capable of receiving the contaminantfrom the gas stream and wherein the positively charged DC outlets areoriented substantially peripheral to the gas stream and normal to theflow of the gas stream; at least a second negatively charged DCelectrode located downstream of the positively charged DC outlets andoriented substantially peripheral to the gas stream and normal to theflow of the gas stream, wherein the plurality of positively charged DCoutlets and the second negatively charged DC electrode cooperativelygenerate a first electric field that imparts a drift velocity to thematerial; and a plurality of charged AC electrodes orientedsubstantially peripheral to the gas stream and normal to the flow of thegas stream, wherein the plurality of charged AC electrodes generate asecond electric field that imparts additional motion to the material.

In another embodiment, a method for receiving contaminants in a gasstream using the VSB system 20 comprises: a) introducing a material intothe gas stream through at least one outlet, wherein the material iscapable of receiving the contaminant from the gas stream; b) generatinga first electric field from at least a first DC charged electrode and atleast a second charged DC electrode, wherein the first electric fieldimparts a drift velocity to the material and wherein the first andsecond DC charged electrodes are oriented substantially peripheral tothe gas stream and normal to the flow of the gas stream; and c)generating a second electric field from at least one charged ACelectrode oriented substantially peripheral to the gas stream and normalto the flow of the gas stream, wherein the second electric field impartsadditional motion to the material. In a further embodiment, the methodcomprises accumulating and collecting the material after the materialhas removed the contaminant from the gas stream.

By way of example and not by limitation, the following additionalembodiments of the VSB system 20 are contemplated.

In an embodiment, any suitable powdered catalysts such as titanium andvanadium could be introduced into the gas stream through the powderedsolid material introducing mechanism 100. For example, the powderedcatalysts can facilitate the use of the VSB system 20 to remove nitrogenoxides from waste gas streams. One or more liquid injectors could beused to disperse ammonia into the gas stream. Preferably, the liquidinjectors should be placed upstream of the charged electrodes a distancesufficient to assure a largely uniform ammonia distribution within thegas stream at the charged electrodes.

In another embodiment, several VSBs could be placed in series with eachVSB facilitating the removal of different trace gas species.

In an alternative embodiment, the VSB system 20 could facilitate theincrease of mass transfer between trace gas species and powdered solidmaterial if the solid material were introduced in bulk and charged witha corona as is typical in electrostatic precipitators.

In an embodiment, the VSB system 20 could facilitate the increase ofmass transfer between trace gas species and powdered solid material ifthe solid material were formed or precipitated in situ upstream of theVSB system 20. For example, a particle could be formed in situ bycondensing a vapor by precipitation or as a by-product of a combustionprocess. The solid material formed in situ could then pass over acharged corona as is typical in electrostatic precipitators.

In another embodiment, the VSB system 20 could be used as part of anintegrated system for detecting chemical and biological warfare (CBW)agents. For example, impedance-based electrochemical sensors detect thepresence of CBW agents by measuring the change in impedance of a thinfilm of water. Biomolecular recognition technology has previouslysuffered from several perceived shortcomings. The fact that biomoleculesoperate only in aqueous environments previously made biosensorsunsuitable for detecting species in the gas phase. Low analyteconcentrations slowed detection due to their effects on the kinetics ofspecific biomolecular recognition interactions. Such characteristicsseverely limited transfer of biosensor technology to practicalapplications.

The VSB system 20 overcomes these obstacles. Using an embodiment of theVSB system 20, the CBW agent is transferred to the liquid phase by anovel, enhanced mass transfer process. The ability to rapidly andefficiently transfer a gas-phase analyte to the liquid phase is a majoradvance over competing technology.

To detect airborne threats, aqueous phase detection devices mustnecessarily transfer the analyte from the gas phase of the sampled airstream to the aqueous film. Conventional gas chromatography relies ongaseous diffusion to affect this mass transfer process. However, becauseFickian gas diffusion rates are proportional to the concentrationgradient, diffusive mass transfer rates are extremely slow for traceanalyte concentrations, such as would be expected for CBW agents.Bench-top gas chromatography addresses this issue by using long,narrow-bore tubes to provide long residence times and short diffusiondistances. Such features are impractical if compact packaging, highthroughput, low power consumption, and near-real-time detection aredesired.

In an embodiment, the VSB system 20 is well-suited for such challenginggas-liquid mass transfer tasks. For example, the VSB system 20 iscapable of removing part-per-billion concentrations of elemental mercuryfrom coal combustion exhaust gases. In another embodiment, the VSBsystem 20 introduces a charged aerosol sorbent into the target gasstream. The suspended aerosol is then preferably subjected to twomutually orthogonal electric fields: a) a DC field that induces aconstant, cross-stream drift velocity on each sorbent particle, and b)an AC field that superimposes an orthogonal, sinusoidal velocitycomponent. Adapting the VSB system 20 for highly efficient gasseparation for CBW agent detection holds significant promise. In anembodiment, the VSB system 20 is adapted for CBW agent detection mightuse a liquid aerosol of atomized water droplets. Further, in analternative embodiment, the VSB system 20 uses electric fields tomanipulate charged aerosols offering exceptional opportunities forminiaturization. Because electric field strength varies inversely withcharacteristic dimension, the miniaturization desired of Micro GasAnalyzers will reduce the voltage requirements and power consumptionassociated with the VSB system 20.

In an embodiment, the VSB system 20 may be adapted for use with anaqueous phase detection device. For example, a gas stream extracted fromthe monitored volume of air first undergoes humidification by injectinga simple water mist from a prior art flush-mounted piezoelectricatomizer. Such piezoelectric atomizers are commonly found in householdair humidifiers and easily produce fine mists of droplets with diameterson the order of 10 μm. The production of so many droplets of such smallsize provides a tremendous total surface area for adsorption of theanalyte. As the mist evaporates, the gas stream becomes nearly saturatedwith water vapor (relative humidity ˜100%). After the humidificationprocess, a second array of piezoelectric atomizers injects a fine mistof charged water droplets. These charged droplets do not evaporate inthe nearly saturated (water vapor) gas stream. These charged waterdroplets adsorb species from the gas-phase as they trace a sinuous pathacross the gas stream, drawn by the two mutually orthogonal AC and DCelectric fields. After traversing the gas stream, the charged dropletsimpact the grounded plate electrode, lose their charge, and arecollected. The collected, uncharged liquid is then directed to theaqueous phase detection device for detection and discrimination of CBWagents.

In an embodiment, the VSB system 20 exposes the gas to the exceptionallylarge surface area of the suspended aerosol. The three-dimensionalmotion induced in the dispersed phase by the electric fields insures acontinuous high relative velocity between the two phases even as theaerosol is entrained in the gas flow. The product of the interphaserelative velocity (m/s) and the exceptionally large adsorption surfacearea of the aerosol (m²) yield a very high swept volume rate (m³/s) thathas a first-order effect on adsorption rate. The VSB system 20preferably provides compact, low power mass transfer. Because the gaschromatographic approach of small bore columns is not used, VSBs presentnegligible additional pressure drop within the gas flow. The twoelectric fields consume little power due to the small flow of currentbetween the electrodes, and the required voltage can be attained usingsolid state transformers. The VSB system 20 as described is well-suitedfor passive and nearly maintenance-free operation, only requiringelectric power and a small supply of water for humidification. The waterflows, electrostatic voltages and frequencies are all variable, allowingthe system to be programmed to respond in real time to detection events.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its intended advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

1. A system comprising: at least one outlet for introducing a materialinto a gas stream, wherein the material is capable of receiving acontaminant from the gas stream; at least a first charged DC electrode;at least a second charged DC electrode, wherein the first DC chargedelectrode and the second charged DC electrode cooperatively generate afirst electric field that imparts a drift velocity to the material; anda plurality of charged AC electrodes, each charged AC electrode orientedsubstantially peripheral to the gas stream and normal to the flow of thegas stream, wherein each charged AC electrode generates a secondelectric field that imparts additional motion to the material, andwherein the second electric field is orthogonal to the first electricfield.
 2. The system of claim 1, wherein the material is electricallycharged prior to entering the gas stream.
 3. The system of claim 1,wherein the first charged DC electrode and the second charged DCelectrode have a different voltage.
 4. The system of claim 1, whereinthe second charged DC electrode has voltage of 0 and is grounded.
 5. Thesystem of claim 1, wherein the outlet comprises the first charged DCelectrode.
 6. The system of claim 1, wherein the second chargedelectrode comprises a plate so constructed and arranged for collectingthe material.
 7. The system of claim 1, wherein the at least one outletcomprises a plurality of outlets that are stacked.
 8. The system ofclaim 1, wherein the at least one outlet comprises a plurality ofoutlets that are in series along the gas stream.
 9. The system of claim1, wherein the motion is periodic.
 10. The system of claim 1, whereinthe material is a solid material selected from the group consisting of asorbent, a catalyst and combinations thereof.
 11. The system of claim 1,wherein the material is capable of receiving a plurality of contaminantsfrom the gas stream.
 12. The system of claim 1, wherein the outlet isconstructed and arranged for injecting a liquid into the gas stream. 13.The system of claim 12, wherein the outlet is located upstream of thefirst charged DC electrode.
 14. The system of claim 12, wherein theinjected liquid is selected from the group consisting of an ammoniasolution, a urea solution, an aerosol and combinations thereof.
 15. Avirtual sorbent bed system for receiving a contaminant from a gasstream, the system comprising: a plurality of positively charged DCoutlets for introducing a material into the gas stream, wherein thematerial is capable of receiving the contaminant from the gas stream andwherein the positively charged DC outlets are oriented substantiallyperipheral to the gas stream and normal to the flow of the gas stream;at least a second negatively charged DC electrode located downstream ofthe positively charged DC outlets and oriented substantially peripheralto the gas stream and normal to the flow of the gas stream, wherein theplurality of positively charged DC outlets and the second negativelycharged DC electrode cooperatively generate a first electric field thatimparts a drift velocity to the material; and a plurality of charged ACelectrodes oriented substantially peripheral to the gas stream andnormal to the flow of the gas stream, wherein the plurality of chargedAC electrodes generate a second electric field that imparts additionalthree-dimensional motion to the material.
 16. The system of claim 15,wherein the material is selected from the group consisting of a solidmaterial, a liquid material, a powdered material, an aerosol, a sorbent,a catalyst and combinations thereof.
 17. The system of claim 16, whereinthe material is capable of receiving a plurality of contaminants fromthe gas stream.
 18. The system of claim 15, wherein the material iselectrically charged prior to entering the gas stream.